Method of immediately terminating free-radical polymerisation processes

ABSTRACT

Free radical polymerizations immediately terminated by a process comprising the addition of an inhibitor solution which contains phenothiazine and at least 50% (w/w) N-alkylpyrrolidone to a system undergoing free radical polymerization, wherein the inhibitor solution also contains p-methoxyphenol (MEHQ).

The present invention relates to a process for the immediate terminationof free radical polymerizations. Once initiated, free radicalpolymerizations are usually highly exothermic, ie. take place withconsiderable evolution of heat, the heat of polymerization, unlessremoved, additionally accelerating the free radical polymerization.

If the abovementioned heat removal takes place in an inadequate mannerin intentional free radical polymerizations, there is a danger that thepolymerization will be so vigorous that the vessel containing thepolymerization mixture will explode if the runaway polymerization (forexample, free radical mass, solution, emulsion or suspensionpolymerization of compounds (monomers) having at least one ethylenicallyunsaturated group) is not counteracted.

Such an effective countermeasure is also required in particular in thecase of unintentionally initiated free radical polymerizations.Unintentionally initiated free radical polymerizations may occur, forexample, during the storage and/or transport of substances containingmonomers, since both heat and light or undesired free radicals caninitiate the free radical polymerization of monomers. It is true that anattempt is usually made to prevent such unintentional free radicalpolymerizations by adding small amounts (as a rule up to 1000 ppm byweight) of free radical polymerization inhibitors (free radicalacceptors, polymerization inhibitors) to the monomers. However, theirinhibiting effect must not be too pronounced since otherwise they wouldhave to be separated off before subsequent use of the monomers for freeradical polymerization purposes. However, free radical polymerizationinitiators can usually predominate over a moderately inhibiting effect,as possessed, for example, by the monomethyl ether of hydroquinone(p-methoxyphenol-MEHQ), and it is for this reason that MEHQ is a storageand/or transport stabilizer particularly frequently used for monomers.However, experience has shown that, even in the case of monomersstabilized with storage and/or transport stabilizers, an unintentionalfree radical polymerization of said monomers cannot be completely ruledout. The latter applies in particular when the monomers are(meth)acrylic monomers and/or styrene, which particularly readilyundergo free radical polymerization.

The term (meth)acrylic monomers is to be understood here as meaningsubstances which comprise acrolein, methacrolein, acrylic acid,methacrylic acid and/or esters of the two abovementioned acids. In thispublication, (meth)acrylic is generally used as an abbreviation foracrylic and/or methacrylic.

Especially substances which comprise at least 90% by weight of(meth)acrylic monomers and/or styrene are at risk with regard to anunintentional free radical polymerization (this also applies whenpolymerization inhibitors are added as a preventive measure). Thisapplies in particular when such substances are exposed to extremeexternal conditions during transport and/or during storage (for example,extremely high temperatures during transport by ship through variousclimatic zones (eg. across the equator), as is the case, for example,for transport from Europe to Southeast Asia, or extremely lowtemperatures, as in the case of storage in outdoor tanks in northerncountries). In particular, low temperatures are not without risks sincethey can in extreme cases lead to partial or complete crystallization ofthe monomers. The latter usually results in separation of monomers andstabilizer (purification by crystallization), which may lead to thepresence of unstabilized regions of monomers and subsequent melting fora certain duration, which regions can with high probability be thestarting point of an unintentional free radical polymerization.

For safe transport and/or safe storage of monomer-containing substances,there is therefore a need for a process which is capable of very rapidlyterminating an unintentionally initiated free radical polymerization ofthe monomers. Such a process is also required, however, for stoppingrunaway unintentional free radical polymerizations immediately.

The processes known to date which envisage the addition of a knowninhibitor solution are very unreliable at low temperatures owing to thecrystallization of the inhibitor solution.

It is an object of the present invention to provide a process forimmediately terminating free radial polymerizations which is alsoapplicable at temperatures below 0° C.

We have found that this object is achieved by a process for immediatelyterminating free radical polymerizations, comprising the addition of aninhibitor solution which contains phenothiazine and at least 50% (w/w)N-alkylpyrrolidone to a system undergoing free radical polymerization.

The object is achieved, according to the invention, if the inhibitorsolution also contains p-methoxyphenol (MEHQ).

The advantageous nature of the novel process is a result of intensiveand extensive research activity, as follows:

compared with the recommendation of EP-B 64628 and EP-A 200181 to add apolymerization inhibitor solution based on hydroquinone orbutylpyrocatechol or derivatives thereof for immediately terminating afree radical polymerization, the inhibitor solution to be addedaccording to the invention containing phenothiazine contains asubstantially more efficient and more broadly applicable free radicalpolymerization inhibitor;

compared with the recommendation of Res. Dicl. 1989, 300, 245 (Eng.) toadd an aqueous Cu(II) salt solution for immediately terminating anunintentional free radical polymerization of acrylic acid, inhibitorsolutions based on N-alkylpyrrolidone are as a rule, on the one hand,miscible both with aqueous and with nonaqueous systems and, on the otherhand, also subsequently readily separable from such systems;

Process Saf. Prog. 12(2) (1993), 111-114 recommends, for immediatelyterminating an unintentional free radical polymerization of acrylicacid, adding thereto an inhibitor solution based on phenothiazine, butthis prior art contains no indication that the phenothiazine is to beadded in solution in a solvent comprising mainly N-alkylpyrrolidone.

PCT/EP98/06814 of Oct. 27, 1998 describes a process for immediatelyterminating free radical polymerizations, comprising the addition of aninhibitor solution which contains phenothiazine and at least 50% (w/w)N-alkylpyrrolidone to a system undergoing free radical polymerization.However, the resulting inhibitor solution crystallizes close to 0° C. Itis not mentioned that the addition of p-methoxyphenol (MEHQ) to theinhibitor solution would be advantageous. By adding MEHQ to theinhibitor solution, however, the crystallization point of the inhibitorsolution is dramatically lowered so that the inhibitor solution can nowalso be reliably used at temperatures below 0° C.

The crystallization-inhibiting properties of MEHQ to this extent arecompletely surprising. To date, MEHQ has been added only as a storageand/or transport stabilizer to the monomers capable of free radialpolymerization.

Further advantages of the novel procedure are that N-alkylpyrrolidonesare inert to most substances. Furthermore, the boiling point ofN-alkylpyrrolidones is above the boiling point of most monomers,facilitating subsequent separation from the monomers and permittingsubsequent further use of the monomers. Moreover, the high boiling pointof the N-alkylpyrrolidones prevents the formation of explosivevapor/oxygen mixtures in hot climatic zones. Furthermore,N-alkylpyrrolidones generally have a low melting point, which permitstheir use even in northern countries. Another advantage is the lowflashpoint of N-alkylpyrrolidones and their low toxicity, if they aretoxic at all. However, the fact that phenothiazine has high solubilityin N-alkylpyrrolidone at room temperature (25° C.) is very particularlyadvantageous for the novel process. This permits the novel use ofphenothiazine solutions having a high phenothiazine content without therisk of the phenothiazine being immediately partially or completelyprecipitated from the solution with a change of outdoor temperature.

The addition of phenothiazine as such for immediately terminating freeradical polymerizations is a disadvantage in that the low degree ofdivision of the phenothiazine as such is not appropriate for therequired immediate termination. N-alkylpyrrolidones preferred accordingto the invention are those with alkyl groups of 1 to 8 carbon atoms.Particularly preferred among these are the N-alkylpyrrolidones whosealkyl group is of 1 to 6 carbon atoms. Very particularly preferredN-alkylpyrrolidones are N-methylpyrrolidone and N-ethylpyrrolidone. Inaddition to N-alkylpyrrolidones, the phenothiazine solution to be addedaccording to the invention may also contain other solvents. Suitablesolvents of this type are all those which are miscible withN-alkylpyrrolidones. Examples of such solvents are biphenyl, diphenylether, toluene, xylene, dimethyl phthalate, butyl acetate and2-ethylhexyl acetate. N,N-dialkylcarboxamides whose alkyl groups arepreferably of 1 to 8 carbon atoms are furthermore suitable as such othersolvents. Particularly advantageous alkyl groups are methyl, ethyl andn-butyl. N,N-dialkylcarboxamides of C₁-C₃-alkanecarboxylic acids arealso particularly advantageous. N,N-dialkylcarboxamides particularlyadvantageous according to the invention are N,N-dimethylformamide andN,N-dimethylacetamide.

The solvent of the phenothiazine solution to be added according to theinvention preferably comprises at least 75%, particularly preferably atleast 85%, very particularly preferably at least 95%, based on theweight of said solvent, of N-alkylpyrrolidone. According to theinvention, the solvent of the phenothiazine solution advantageouslyconsists exclusively of N-alkylpyrrolidone, in particular exclusively ofN-methylpyrrolidone or exclusively of N-ethylpyrrolidone.

In addition to phenothiazine and MEHQ, the inhibitor solution to beadded according to the invention may also contain other polymerizationinhibitors. Examples of these are hydroquinone, diphenylamine,p-phenylenediamines, nitroxyl radicals (compounds which have at leastone >N-O*-group), compounds which have a nitroso group, ie. a group—N═O, and hydroxylamines.

Nitroxyl radicals (also referred to as N-oxyl radicals) which areparticularly suitable according to the invention are those derived froma secondary amine which carries no hydrogen atoms on the a-carbon atoms(ie. the N-oxyl groups are derived from corresponding secondary aminogroups). Particularly suitable among these are the N-oxyl radicals whichare stated in EP-A 135280, the prior application DE-A 19651307, U.S.Pat. No. 5,322,912, U.S. Pat. No. 5,412,047, U.S. Pat. No. 4,581,429,DE-A 1618141, CN-A 1052847, U.S. Pat. No. 4,670,131, U.S. Pat. No.5,322,960, the prior application DE-A 19602539, EP-A 765856 and JP-A5/320217.

Such suitable, stable N-oxyl radicals derived from a second amine are,for example, those of the general formula I

where

R¹, R², R⁵ and R⁶ are the same or different straight-chain or branched,unsubstituted or substituted alkyl groups and

R³ and R⁴ are the same or different straight-chain or branched,unsubstituted or substituted alkyl groups or

R³CNCR⁴ is an unsubstituted or substituted, cylic structure.

Compounds I which are particularly suitable according to the inventionare those which are stated in EP-A 135 280, the prior application DE-A19651307, U.S. Pat. No. 5,322,912, U.S. Pat. No. 5,412,047, U.S. Pat.No. 4,581,429, DE-A 16 18 141, CN-A 1052847, U.S. Pat. No. 4,670,131,U.S. Pat. No. 5,322,960 and the prior application DE-A 19602539.

Examples of these are the stable N-oxyl radicals of the general formulaI where R¹, R², R⁵ and R⁶ are (identical or different) C₁-C₄ alkylgroups, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl ortert-butyl, linear or branched pentyl, phenyl or a substituted groupthereof and R³ and R⁴ are (identical or different) C₁-C₄ alkyl groups,such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl ortert-butyl, linear or branched pentyl or a substituted group thereof or,together with CNC, are the cyclic structure

where n is an integer from 1 to 10 (frequently from 1 to 6), includingsubstituted cyclic structures of this type. Typical examples are2,2,6,6-tetramethyl-1-oxylpiperidine,2,2,5,5-tetramethyl-1-oxylpyrrolidine and4-oxo-2,2,6,6-tetramethyl-1-oxylpiperidine.

The N-oxyl radicals I can be prepared from the corresponding secondaryamines by oxidation, for example with hydrogen peroxide. As a rule, theycan be prepared as pure substance.

The N-oxyl radicals I suitable according to the invention include inparticular piperidine- or pyrrolidine-N-oxyls and di-N-oxyls of thefollowing general formulae II to IX:

where

m is from 2to 10,

R⁷, R⁸ and R⁹, independently of one another, are each

M⁶¹ is a hydrogen ion or an alkali metal ion,

q is an integer from 1 to 10,

R¹, R^(2,), R^(5,), R^(6,), independently of one another, andindependently of R¹, R², R⁵ and R⁶, are the same groups as R¹,

R¹⁰ is C₁-C₄-alkyl, —CH═CH₂, —C≡CH, —CN,

—COO^(⊖)M^(⊕), —COOCH₃ or —COOC₂H₅,

R¹¹ is an organic radical which has at least one primary, secondary (eg.—NHR¹) or tertiary amino group (eg. —NR¹R²) or at least one ammoniumgroup —N^(⊕)R¹⁴R¹⁵R¹⁶X^(⊖), where X^(⊖)is F^(⊖), Cl^(⊖), Br^(⊖), HSO₄,SO₄ ², H₂PO₄, HPO₄ ² or PO₄ ³ and R¹⁴, R¹⁵ and R¹⁶ are organic radicalsindependent of one another (eg. independently of one another andindependently of R¹, are the same groups as R¹),

R^(12,) , independently of R¹¹, is one of the same groups as R¹¹ or —H,—OH, C₁-C₄-alkyl, —COO^(⊖)M^(⊕), —C≡CH,

or hydroxyl-substituted C₁-C₄-alkyl (eg. hydroxyethyl or hydroxypropyl)or

R¹¹ and R¹² together are the oxygen of a carbonyl group and

Preferably, R¹═R²═R⁵═R⁶═R^(1′)═R^(2′)═R^(5′)═R^(6′)═—CH₃.

Examples of typical N-oxyl radicals suitable according to the inventionare

4-hydroxy-2,2,6,6-tetramethyl-1-oxylpiperidine,

4-hydroxy-2,6-diphenyl-2,6-dimethyl-1-oxylpiperidine,

4-carboxy-2,2,6,6-tetramethyl-1-oxylpiperidine,

4-carboxy-2,6-diphenyl-2,6-dimethyl-1-oxylpiperidine,

3-carboxy-2,2,5,5-tetramethyl-1-oxylpyrrolidine,

3-arboxy-2,5-diphenyl-2,5-dimethyl-1-oxylpyrrolidine,

4-acetyl-2,2,6,6-tetramethyl-1-oxylpiperidine,

N,N′-bis(1oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N′-bisformyl-1,6-diaminohexaneand

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) adipate.

The preparation of 3-carboxy-2,2,5,5-tetramethyl-1-oxyl-pyrrolidine isdescribed, for example in Romanelli, M., Ottaviani, M. F., Martini, G.,Kevan, L., JPCH J: Phys. Chem., EN, 93, 1 (1989), 317-322.

The compounds (VI) and (VII) can be obtained according to U.S. Pat. No.4665185 (eg. Example 7) and DE-A 19510184.

Further suitable typical examples are:

According to the invention, mixtures of an N-oxyl radical can of coursealso be used in addition to phenothiazine.

Organic nitroso compounds suitable according to the invention are, forexample, N-nitrosoarylamines or nitroso compounds having a nitroso groupbonded directly to a carbon atom of an aromatic nucleus. Examples arenitrosophenols, such as 4-nitrosophenol, nitrosonaphthols, such as2-nitroso-1-naphthol, nitrosobenzene, N-nitroso-N-methylurea,nitroso-N,N-dialkylanilines where alkyl is methyl, ethyl, propyl and/orbutyl, N-nitrosodiphenylamine, N-nitrosophenylnaphthylamine,4-nitrosodinaphthylamine and p-nitrosodiphenylamine. According to theinvention, mixtures of the abovementioned nitroso compounds can ofcourse also be used in addition to phenothiazine.

p-Phenylenediamines suitable according to the invention are those of thegeneral formula X

where

R¹⁶, R¹⁷ and R¹⁸, independently of one another, are each alkyl, aryl,alkaryl or aralkyl of up to 20 carbon atoms or hydrogen.

Compounds X where R¹⁶, R¹⁷ and R¹⁸, independently of one another, areeach methyl, ethyl, propyl, isopropyl, isobutyl, sec-butyl, n-butyl,pentyl, phenyl or naphthyl are particularly suitable. Examples ofsuitable compounds X are: N,N′-bis-sec-butyl-p-phenylenediamine,N-phenyl-N′-isopropylphenylenediamine,N-naphthyl-N′-sec-butyl-p-phenylenediamine,N,N,N′-trimethyl-p-phenylene-diamine,N,N,N′-triethyl-p-phenylenediamine, N,N-dimethyl-p-phenylene-diamine,NN-diethyl-p-phenylenediamine,N-phenyl-N′,N′-dimethyl-p-phenylenediamine,N-phenyl-N′,N′-diethyl-p-phenylenediamine,N-phenyl-N′,N′-dipropyl-p-phenylenediamine,N-phenyl-N′,N′-di-n-butyl-p-phenylene-diamine,N-phenyl-N′,N′-di-sec-butyl-p-phenylenediamine,N-phenyl-N′-methyl-N′-ethyl-p-phenylenediamine,N-phenyl-N′-methyl-N′-propyl-p-phenylenediamine,N-phenyl-N′-methyl-p-phenylenediamine,N-phenyl-N′-ethyl-p-phenylenediamine,N-phenyl-N′-propyl-p-phenylenediamine,N-phenyl-N′-isopropyl-p-phenylenediamine,N-phenyl-N′-butyl-p-phenylenediamine,N-phenyl-N′-isobutyl-p-phenylenediamine,N-phenyl-N′-sec-butyl-p-phenylenediamine,N-phenyl-N′-tert-butyl-p-phenylenediamine,N-phenyl-N′-n-pentyl-p-phenylenediamine,N-phenyl-N′-n-hexyl-p-phenylenediamine,N-phenyl-N′-(1-methylhexyl)p-phenylenediamine,N-phenyl-N′-(1,3-dimethylbutyl)p-phenylenediamine, N-phenyl-N′-(1,4-dimethylpentyl)-p-phenylenediamine and p-phenylenediamine. Accordingto the invention, mixtures of p-phenylenediamines may also be used inaddition to phenothiazine. Particularly suitable mixtures of this typeare the p-phenylenediarnine mixtures recommended in WO 92/01665.

According to the invention, mixtures of all different abovementionedpolymerization inhibitors may of course also be used in addition tophenothiazine and MEHQ.

Phenothiazine solutions which are preferred according to the inventionare those whose total content of polymerization inhibitor comprises atleast 50, particularly preferably at least 75, very particularlypreferably at least 90, % by weight of phenothiazine. Particularlyadvantageously, no further polymerization inhibitor apart fromphenothiazine and MEHQ is present in the inhibitor solution to be addedaccording to the invention. MEHQ scarcely acts as an inhibitor in theinhibitor solution.

As a rule, the content of phenothiazine in the inhibitor solutions to beused according to the invention is at least 10, preferably at least 20,particularly preferably at least 30, % (w/w), based on the solution.

According to the invention, a solution of phenothiazine inmethylpyrrolidone, whose phenothiazine content is advantageously from 35to 45%, based on the solution, is preferred. Frequently, thephenothiazine content of the abovementioned solution is 35% (w/w).

The content of the crystallization-inhibiting MEHQ in the inhibitorsolution is preferably from 2.5 to 12.5% (w/w), in particular from 5 to10% (w/w), based on the weight of the inhibitor solution.

In particularly preferred embodiments of the invention, the inhibitorsolution comprises 35% of phenothiazine, 5% of MEHQ and 60% ofN-methylpyrrolidone or 30% of phenothiazine, 10% of MEHQ and 60% ofN-methylpyrrolidone (in each case w/w).

The novel process is suitable for immediately terminating any type offree radical polymerizations even below 0° C., in particular thoseunintentional and/or runaway free radical polymerizations mentioned atthe beginning of this publication.

These include in particular the unintentional free radicalpolymerizations of those substances which comprise at least 95 or atleast 98 or at least 99 or 100% by weight of (meth)acrylic monomers.Particularly suitable (meth)acrylic monomers are (meth)acrylic acid andesters or (meth)acrylic acid and monohydric or polyhydric alcohols. Thisapplies in particular when the monohydric or polyhydric alkanols are ofone to twenty carbon atoms or one to twelve carbon atoms or one to eightcarbon atoms. Typical examples of such esters are for example methylacrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate,tert-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethylmethacrylate, n-butyl methacrylate and tert-butyl methacrylate.

The invention furthermore relates to an apparatus for carrying out anovel process.

It is expedient in terms of application technology to introduce theinhibitor solution to be added according to the invention via a spraynozzle in order to achieve very rapid homogeneous distribution in thesystem undergoing free radical polymerization. The apparatuses describedin DE-A 197 49 859 and DE-A 198 22 492 are particularly suitable forintroducing the inhibitor solution into the system undergoingpolymerization.

Of course, the abovementioned homogenization can also be supported bycirculation by means of a pump and/or by stirring. However, suchmechanical auxiliary measures also entail the danger of acceleration ofthe polymerization since they are also associated with introduction ofenergy into the system undergoing free radical polymerization. Thephenothiazine solution to be introduced is expediently contained in asuitable storage container. If the novel process for immediatelyterminating the free radical polymerization of (meth)acrylic monomersunintentionally polymerizing in the absence of a solvent, the totaladded amount of phenothiazine should be from about 0.01 to 3% by weight,based on the (meth)acrylic monomers. As a rule, an added amount of from0.01 to 0.05, frequently 0.025, % by weight of phenothiazine issufficient.

EXAMPLE

Composition of the inhibitor solution Crystallization point 35% (w/w)phenothiazine  +2° C. 65% (w/w) N-methylpyrrolidone 35% (w/w)phenothiazine −11° C.  5% (w/w) MEHQ 60% (w/w) N-methylpyrrolidone

Comparison of the crystallization point of an inhibitor solutioncontaining MEHQ with that of an inhibitor solution without MEHQ showsthat MEHQ lowers the crystallization point and extends the operatingrange by 13° C., from 2° C. to −11° C. The effect of phenothiazine as apolymerization inhibitor is not impaired.

We claim:
 1. A process for immediately terminating free radicalpolymerizations comprising the addition of an inhibitor solution whichcontains phenothiazine and at least 50% (w/w) N-alkylpyrrolidone—basedon the weight of said inhibitor solution—to a system undergoing freeradical polymerization, wherein the inhibitor solution also contains“2.5 to 12.5% (w/w)” p-methoxyphenol.
 2. The process as claimed in claim1, wherein the N-alkylpyrrolidone is N-methylpyrrolidone orN-ethylpyrrolidone.
 3. The process as claimed in claim 1, wherein thephenothiazine content of the inhibitor solution is at least 10% (w/w)based on the weight of said inhibitor solution.
 4. The process asclaimed in claim 1, wherein the p-methoxyphenol content of the inhibitorsolution is from 5 to 10% (w/w), based on the weight of said inhibitorsolution.
 5. The process as claimed in claim 1, wherein thephenothiazine content of the inhibitor solution is at least 10% (w/w)based on the weight of said inhibitor solution.
 6. The process asclaimed in claim 2, wherein the phenothiazine content of the inhibitorsolution is at least 10% (w/w) based on the weight of said inhibitorsolution.
 7. The process as claimed in claim 1, wherein the systemundergoing free radical polymerization comprises (meth)acrylic monomersundergoing free radical mass polymerization.
 8. The process as claimedin claim 7, wherein the (meth)acrylic monomer is (meth)acrylic acid. 9.The process as claimed in claim 7, wherein the (meth)acrylic monomer isa (meth)acrylic ester.
 10. An inhibitor solution, containing at least10% (w/w) phenothiazine, from 5 to 10% (w/w) p-methoxyphenol and atleast 50% (w/w) N-methylpyrrolidone, in each case based on the weight ofsaid inhibitor solution.